Simply: Unblock them. Because if you have someone blocked they will not be able to join the twitter group chat. So don't be that person or you just won't be allowed in.
Twitter is where both the ceremony and reception will take place. The ceremony will have its own group chat and the reception will have another. Once the ceremony has finished people will move to the reception group chat. Aka the first group chats location is the Hotel and the second group chats location is on the cruise. If you have any questions, just ask.
After the ceremony and reception, there will be a rabbit that will act as the hotel bar/lobby where Baek and Yeol will go back to chat with people before they leave Paris and part ways for their vacation. People who wish to go can just message Baek for his rabbit.
Anyone who has offered to sing/perform at the wedding will do so at the reception after gifts and speeches. The order of who is going will be decided the morning of the wedding, just ask if you're unsure about anything.